In the area of computer graphics, often a lightpen is used to detect the presence of the swept beam of a Cathode Ray Tube (CRT) within the optical aperture of the lightpen. This event, generally recorded by a solid state sensor in the pen, is amplified by appropriate electronics, and an interrupt signal is sent to the computer system that controls the display process. This interrupt causes certain software to be executed within the computer in response to the lightpen strike or hit. Such software generally interrogates appropriate registers in the display processor so that the associated address of data in a display list is found, together with general status information such as present beam location expressed in a set of orthogonal coordinates.
The lightpen requires a threshold amount of light energy to excite the detection electronics. This energy is derived from the energy conversion process that takes place in the CRT phosphor, where the kinetic energy of the beam electrons is transformed to light output from the phosphor. This process is not instantaneous, and it creates a delay in producing the light needed to energize the pen. The delay is largely a function of the particular phosphor or phosphor mix used in the CRT. Modern displays move the beam at a high linear velocity. There is inevitably a significant delay between the entry of the electron beam into the area of the lightpen aperture, and the generation of adequate light energy from that area of the CRT. This energy must be transferred to and detected in the pen itself, and the event registered in the computer as a "lightpen interrupt". Each of these processes creates a delay in recording the pen hit. On pen strikes well within long vectors displayed on the CRT this is of little consequence because the strike will be registered some time after the beam has passed through the pen aperture, and the display list address and display status information will be correct. Useful pen information will be derived even if the associated orthogonal coordinates location information is somewhat outside the beam aperture.
Problems arise when the pen hit is towards the end of a screen vector. When a vector is ended, the CRT beam is switched off, and the display processor goes into an INTERPRET mode during which the display processor and possibly the host computer decide how to handle the next display instruction. This may typically be another vector, a character, a reset position command, etc. The arrival of a lightpen interrupt during this INTERPRET phase when the beam is off generally gives rise to problems in that it is generally inconvenient to interrupt the interpretation process, and it is complex to develop a full set of algorithms in hardware or software to reliably process such pen-hit data. One known solution is to reject or ignore the "hit". However, rejecting or ignoring the hit may have the effect of making it impossible to interact, by means of a lightpen, with a displayed pattern that consists of a large number of very short vectors, as is typically the case with a circle. This is a drawback existing in several commercially available graphics systems.
Therefore, there exists a need for a computer graphics system capable of detecting lightpen interrupts or hits on contiguous vectors having drawing periods shorter than the response time of the lightpen thereby exhibiting maximum use of the information furnished by lightpen hits.